In order to save fuel it is known to stop the internal combustion engine of a motor vehicle when it is not needed and to restart the engine upon driver demand. An engine operated in such a manner is often referred to as being fitted with automatic stop/start control and such vehicles are sometimes referred to as micro-hybrids.
Stop/start control automatically stops and starts the engine when one or more predetermined vehicle operating conditions are met. For example, if the motor vehicle is sensed to be stationary and an accelerator pedal used to provide a driver input is not depressed for a pre-determined period of time, then this may constitute a vehicle operating condition indicating that the engine can be temporarily stopped. If the accelerator pedal is then subsequently depressed, this can be used as a vehicle operating condition to indicate that the engine must be restarted. It will be appreciated that numerous vehicle operating conditions can be used to indicate that the engine can be temporarily stopped or restarted and the above is merely one example.
A technology that can be used to restart the engine is known as a belt driven integrated starter generator (BISG) and is described in detail in US Patent publication 2004/0206325. The integrated starter/generator is used to motor the crankshaft of the engine to start the engine in lieu of a starter motor when the engine needs to be restarted and this leads to considerable loads in the belt drive used to connect the integrated starter/generator (ISG) to the crankshaft of the engine. To prevent excessive belt wear, audible squeal, and loss of torque transmission, it is important to have a mechanism that detects the onset of belt slip. The motoring torque of the BISG machine during an engine cranking event can then be reduced to protect the belt from damage, prolong the service life of the belt, prevent audible squeal, and loss of torque transmission.
It is known to determine if belt slip is occurring by comparing a measured engine speed with a measured motor speed and if there is a difference using this as an indication that belt slip is occurring. The crankshaft speed of the engine is normally measured using a speed sensor which provides a data stream with a high repetition frequency over a data bus to the ISG machine. The ISG machine either has an internal sensor that directly measures the rotor or shaft speed of the ISG or the shaft speed is inferred with high precision by the control system that activates the phase windings of the 3-phase motor.
It is a disadvantage of this technique that the resulting data stream (kbits/second) from the engine speed sensor is so large that an expensive data bus system such as a CAN bus is needed to transport the signal to the ISG machine.
It is a further disadvantage that a missing tooth speed sensor of the type normally used to provide a speed feedback to an engine management control unit is unable to provide a sufficiently high accuracy speed signal for robust belt slip detection and cannot provide a usable signal below a predetermined engine speed such as 150 RPM. Although it is possible to increase the accuracy of the data produced from such a sensor by increasing the number of teeth this will require, additional processing power produces an even larger data stream and is still unable to produce a signal at very low engine speeds.
Accordingly, this invention is directed to providing an improved method for detecting belt slip that can be applied in a cost effective manner, wherein a method for detecting slippage between an electric machine and a machine drivingly connected to the electric machine by a drive belt, comprises: determining at least two operational parameters selected from a rotational speed of a rotor of the electric machine, a current supplied to or generated by the electric machine and a voltage supplied to or generated by the electric machine; and determining whether belt slip is occurring based only upon said determined at least two operational parameters.
The rotational speed of the rotor of the electric machine, the current supplied to or generated by the electric machine, and the voltage supplied to or generated by the electric machine are all determined, and whether belt slip is occurring may be determined based upon at least two or all three operational parameters.
The electric machine may be an electric motor, and determining the rotational speed of the rotor of the electric machine, the current supplied to or generated by the electric machine, and the voltage supplied to or generated by the electric machine may comprise determining the rotational speed of a rotor of the motor, the current supplied to the motor, and the voltage supplied to the motor.
Determining the rotor speed, the current supplied to the motor, and the voltage supplied to the motor may comprise measuring the rotor speed, the current supplied to the motor, and the voltage supplied to the motor.
The method may further comprise reducing at least one of the current and the voltage supplied to the motor in order to reduce the torque produced by the motor when belt slip is determined to be occurring.
Belt slip may be determined to be occurring when the belt slip value is greater than a predetermined threshold.
The electric machine may alternatively be an electric generator.
The electric machine may be an electric generator and determining the rotational speed of the rotor of the electric machine, the current supplied to or generated by the electric machine, and the voltage supplied to or generated by the electric machine may comprise determining the rotational speed of a rotor of the generator, the current generated by the generator, and the voltage generated by the generator.
Determining the rotational speed of the rotor of the generator, the current generated by the generator, and the voltage generated by the generator may comprise measuring the rotational speed of the rotor, the current generated by the generator, and the voltage generated by the generator.
The method may further comprise, when belt slip is determined to be occurring, reducing at least one of the current and the voltage generated by the generator in order to reduce the torque required to rotate the rotor of the generator.
The machine to which the electric machine is drivingly connected may be an internal combustion engine.
It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompanying claims.